Downhole compressor

ABSTRACT

A downhole dynamic compressor comprises an electric motor having a stator  6  with stationary windings and an armature with permanent magnets  7  supported on gas bearings  9, 10  for rotation relative to the stator. The gas bearings  9  and  10  are arranged at the upstream and downstream opposite ends of the motor, respectively. The dynamic compressor has a bladed wheel  12  mounted on an overhanging end of the motor armature  7  that projects beyond the gas bearing  9  at the upstream end of the motor, whereby all the gas bearings of the compressor and the electric motor are arranged on the downstream side of the dynamic compressor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims priority from PCT/GB03/00149,filed on Jan. 15, 2003, which is based on and claims priority fromBritish Application 0200864.7 filed on Jan. 16, 2002, the entiredisclosure of each of the aforementioned applications are each hereinincorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention generally relates a downhole compressor, and morespecifically to a compressor designed to be lowered into a well of anatural gas reservoir to assist in extracting gas from the reservoir.

(2) Description of Related Art

It is known in the art that the gas flowing from a well drilled into agas reservoir frequently carries with it a burden of vapor and liquiddroplets. The pressure of gas at the base of a well falls as gas isextracted. Consequently the flow velocity of the gas in the productiontubing also falls, and eventually becomes too low to carry its burden ofcondensed liquids. As a result, liquid accumulates at the base of thewell, the gas flow falls and eventually ceases. Gas production ceases tobe economically effective before the gas flow ceases and operators willnormally abandon a well long before the gas supply is exhausted.

It has previously been proposed in the PCT patent application numberWO97/33070 to install into the well an electrically or hydraulicallypowered gas compressor to rest at the bottom of the well. The effect ofthe compressor is to accelerate production and increase the ultimaterecovery from the reservoir. In the first place, the compressor acts toreduce the static pressure at its inlet which increases the pressuredifference between the reservoir and the well, so as to stimulategreater flow. Second, by increasing the gas pressure, the compressorincreases the average density which leads to a reduction in flowvelocity and hence in a reduction in the pressure losses along thelength of the well. A further effect of the compression is to raise thetemperature of the gas and thereby delay condensation of vapor.

Though the latter patent application discloses the concept of what isherein termed a downhole compressor, the compressor that it teaches hasseveral limitations that would make it impracticable. For example, theelectric motor used to drive the rotor shaft carrying the impellers thatcompress the produced gas is connected to the rotor shaft throughgearing which allows the motor to rotate much more slowly than theimpellers. This design is to enable the motor to be oil cooled and oillubricated while air bearings are used to support the shaft carrying theimpellers. However, this presents problems with the maintenance of thereduction gearing which are not addressed in the application.Furthermore, the application gives no details of how the gas bearingssupporting the rotor shaft can be constructed or configured to receivean adequate supply of clean gas, nor does it resolve the rotor dynamicrequirements of a shaft system supported on both gas and liquidlubricated bearings.

Accordingly, a need exists to provide a rotary compressor to overcomethe above-mentioned limitations.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a rotary compressor which is suitable foruse as a downhole compressor in that its gas bearings can be operatedover very prolonged periods without requiring attention and in that itselectric motor is adequately cooled by the produced gas.

In accordance with a first embodiment of the present invention, there isprovided a compressor designed to be lowered into a well of a naturalgas reservoir to assist in extracting gas from the reservoir, thecompressor comprising a casing, a rotor mounted within the casing, anelectric motor for driving the rotor having a stator with windingsstationarily mounted in the casing and an armature formed as part of therotor, and gas bearings supporting the rotor for rotation relative tothe stator, the gas bearing being arranged at the upstream anddownstream opposite ends of the motor, characterized in that a bladedimpeller wheel for compressing the production gas from the reservoir ismounted on an overhanging end of the rotor that projects beyond the gasbearing at one end of the motor, such that all the gas bearings of thecompressor and of the electric motor are arranged on the same side ofthe bladed impeller wheel, and during operation, the production gasflows over and serves to cool the electric motor.

In the present invention, the bladed impeller wheel, herein also termedthe main compressor, is overhung.

The design of the motor rotor with an overhung compressor permits therotor to be made hollow so that it can be better cooled.

In a preferred embodiment of the invention, the main compressor isarranged at the upstream end of the rotor and an auxiliary compressor ismounted on the opposite end of the rotor, the auxiliary compressordrawing gas from downstream of the main compressor and serving to supplythe gas after further pressurization to the bearings of the rotor.

In a second embodiment of the invention, both compressors can beoverhung so that all the bearings are situated axially between the mainand auxiliary compressors.

The auxiliary compressor may itself be an axial compressor or other typeof dynamic compressor. The term “dynamic compressor” is used here toinclude rotary compressors that produce axial and/or radial flow andthus in particular includes both axial, mixed and centrifugalcompressors.

It is envisaged that a purifier may be provided in the intake of theauxiliary compressor to remove particulates or other impuritiessuspended in the produced gas. The purifier may conveniently be aninertial separator.

In the preferred embodiment of the invention, the gas for the gasbearings flows in the opposite direction to the main axial gas flow ofthe produced gas. Though the gas can be discharged into the main flow ofthe produced gas after it has passed through the bearings, it ispreferred to cool the gas by transferring heat from it to the main flowof produced gas, whereupon the gas can be recycled to the bearings bybeing returned to the intake of the auxiliary compressor. In this way,it is possible for the gas supplied to the gas bearings to flowessentially in a closed circuit.

When the gas supplied to the bearings flows in a closed circuitcontaining a purifier, the purifier does not have to be able to removethe particulate matter in all of the produced gas and it is thereforeable to function reliably over prolonged periods of time. In this casethe purifier may even be a simple filter.

Because in the present invention gas always enters and leaves thecompressor axially, it is possible to use a modular approach in which anumber of such compressor modules are close coupled (aerodynamically andelectrically) in tandem. Furthermore modules, and/or a set of modules intandem, may be disposed at various depths in the production tube of awell in order to optimize the upward movement of droplets and inhibitthe condensation of vapor.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is an axial section through a first embodiment of dynamicdownhole compressor,

FIG. 2 is a detail of a second embodiment of the invention shown inaxial section,

FIG. 3 is an axial section through a compressor in accordance with athird embodiment of the invention,

FIG. 4 is a detail of a fourth embodiment of the invention shown inaxial section,

FIGS. 5 a and 5 b are idealized enthalpy-entropy diagrams that refer tothe embodiments of FIGS. 3 and 4,

FIG. 6 a is an axial section through a compressor in accordance with afurther embodiment of the invention, and

FIG. 6 b is a section through the compressor of FIG. 6 a taken along theplane A-A in FIG. 6 a.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, reference numeral 1 designates the production tube of a well,numeral 2 designates the outer shell of a compressor and numeral 3refers to the casing of an electric motor. The casing of the motor isheld concentrically within the shell of the compressor by the fixedblades 4 of the compressor and by the arms of a spider 5.

The motor is a high frequency induction motor and is supplied with highfrequency current via an umbilical that is not shown in the Figure.Typically the speed of the motor is in the range of 20,000 rpm to 50,000rpm. The preferred electric motor has a stator 6 and a permanent magnetarmature or rotor 7 but it would be possible to use an alternative formof induction motor, such as a squirrel cage motor.

The rotor of the compressor, of which the armature of the motor forms apart, is designated 8. The rotor runs in journal bearings 9 and 10, andthrust is taken by a thrust bearing having a collar 11.

The motor drives the wheel 12 of the dynamic compressor which has abladed impeller wheel 13. Upstream of the impeller wheel 13 are theinlet guide vanes 14 that also hold concentrically the segment of aninner casing 15.

The direction of the flow of gas, and the direction, in which thecompressor augments the pressure of the gas, is shown by the arrows inthe Figure.

The compressor is constructed as a module. In FIG. 1, a complete moduleis spanned by A, a next module downstream of A is indicated at B, and Cis an inlet nose fairing to be fitted to a single module or to the firstof a number of coupled modules. The cone D is a diffusing cone to befitted at the exhaust of a module or at the exhaust of the last of anumber of modules connected in tandem, i.e. one after the other in thedirection of gas flow.

FIG. 2 shows a detail of a compressor module that differs from themodule A of FIG. 1 in that it has two compressor stages, i.e. two bladedimpeller wheels 13 a and 13 b. One or more stages may be provided independence upon the duty to be performed, the power of the motor, andwhat is found to be the design optimum in each application.

Gas bearings are used because of the speed of the compressor and becausethey can use as a lubricant a fluid already present, namely the producedgas. Gas bearings offer lower friction than water or oil lubricatedbearings. Rolling element bearings would have too short a lifeexpectancy under the onerous down well conditions.

Since the compressor(s) are likely to be mounted either vertically, orin a near vertical attitude, the journal bearings (designated 9 and 10in FIG. 1) will react little load and hence will most likely be of ahydrostatic type. Such bearings rely on the injection of gas at highpressure to separate the contacting surfaces. This high pressure gas isprovided by the auxiliary compressor once it has achieved a sufficientlyhigh rotational speed.

The thrust bearing (designated 11 in FIG. 1) will carry continuous loadand therefore will be of a hydrodynamic type achieving separation by aself-generated film once the shaft reaches a sufficiently high speed.

During start-up, it is anticipated that rubbing contact will occur inall the bearings until the shaft becomes self supporting on the gasfilms. Such starting will necessitate significant power to overcomefriction and necessitates careful material selection and dimensionalcontrol.

The heat generated by the electrical losses of the motor is removed bypassing the heat to the flow of gas, the produced gas being the solecooling medium available.

An embodiment of the invention that includes gas bearings is illustrateddiagrammatically by FIG. 3. The Figure illustrates a version of themodule that is designated A or B in FIG. 1.

In FIG. 3, the production tube of the well is designate 301, the outershell of the compressor 302, while numerals 303 a and 303 b refers to adouble casing of the motor. The casing of the motor is heldconcentrically within the shell of the compressor by stationary blades304 of the compressor and by the arms of a spider 305. The stator of themotor is shown at 306 and its armature at 307.

The hollow rotor of the compressor, of which the armature of the motoris a part, is designated 308. The rotor runs in the journal bearings309, 310, and thrust is taken by a thrust bearing having a collar 311.

The motor drives the wheel 312 of the dynamic compressor with itsimpeller blades 313. Upstream of the compressor are the inlet guidevanes 314 that also hold concentrically the segment of inner casing 315,and downstream at 304 are the fixed blades.

The compressor propels gas into the principal annular channel X that isthe channel for the main flow of the produced gas, but also into anannular channel Y bounded by the walls 303 a and 303 b of the casing ofthe motor. Annular channel Z is formed by the space between the outercasing 302 of the compressor and the production tube 301. The channel Zis closed at each end by annular plates that fit as closely as ispracticable into the bore of the production tube. The pressure inchannel Z is maintained by ports Z1 substantially at the pressureupstream of the inlet guide vanes 314.

Similarly, the pressure over the face of the compressor wheel 312, andwithin the bore of the rotor, is maintained by ports Z2 substantially atthe pressure upstream of the inlet guide vanes.

The gas that flows through channel Y flows over an extended heattransfer surface at Y1 that by welding, or other method of fixing, is inintimate thermal contact with the inner motor casing 303 a. The gas flowthrough channel Y, and past the extended heat transfer surface, coolsthe stator 6 (within FIG. 1) of the motor.

The extended heat transfer surface may by way of example comprise anumber of fins equally spaced around the circle and extending in aspiral around the inner casing of the motor or axially.

Downstream of the extended heat transfer surface the gas flows via apurifier Y2 into the inlet of the auxiliary dynamic compressor that isillustrated with two stages and is indicated as an assembly at 316.

The auxiliary compressor further compresses gas into the volume U thatis bounded on the left-hand side in FIG. 3 by the journal bearing 310and by the labyrinth gland 318 that is bolted to the bearing to ensureconcentricity.

The pressurized gas enters the journal bearing 310 by such ports as maybe convenient, for example the port shown at 319. The gas enters thejournal and thrust bearing 309 from the volume U, for example via pipesL1 laid between adjacent fins of the extended heat transfer surface Y1as shown in FIG. 6 in response of another embodiment of the invention.The flow path of the pipes L1 is represented in FIG. 3 by a chain-dottedline, which is also designated L1.

It is desirable to preserve thermal symmetry such as would be obtainedby four pipes equally disposed around the circle.

The volumes V and W are in communication via the air gap between thebore of the stator of the motor and its armature and consequently thegas pressures in these volumes will be substantially equal. The volume Vand the volume W or both are connected to channel Z by way of hollowspider arms that are not shown and that are necessary to holdconcentrically the various casings. It is to be noted that because ofthrough spaces such as the spaces between the pads, the pressures to theleft and to the right of a bearing become equalized.

In the designation of gas pressures the flow pressure losses, and othereffects that have a detailed influence upon pressure will not be takenin to consideration.

The pressures will be designated as:—

-   -   P1: the pressure of the gas upstream of the compressor module.        By the connecting passages such as Z1 and Z2 it is also the        pressure in the channel Z, and also the pressure acting upon the        left hand face of the wheel 312, and within the bore of the        rotor 308. Spaces V and W are also at pressure P1 by virtue of        their connection with the channel Z via the hollow spider arms,    -   P2: the pressure downstream of the stator blades 304 and the        pressure in the channel X,    -   P3: the pressure downstream of the inner part of the runner        blades 313. This is the pressure in the channel Y, and the        pressure at the inlet of the auxiliary compressor 316, and    -   P4: the pressure downstream of the auxiliary compressor. P4 is        also the pressure supplied to the bearings 309, 310 and 311.

In operation of the module, the inner part of the runner blades 313together with the auxiliary compressor 316 raise the pressure of the gasfrom the pressure P1 via the pressure P3 to the pressure P4. Gas atpressure P4 flows to the bearings where in essence it is throttled inits escape in to the volumes V and W down to the pressure P1. In asimilar fashion the gas leaking through the labyrinth seal 318 isthrottled from the pressure P4 down to the pressure P1.

The axial forces that act upon the rotor during operation are:

a thrust force from right to left (as viewed in FIG. 3) generated by thewheel 312 and the runner blades 313 of the main compressor,

a thrust force from left to right generated by the auxiliary compressor316,

the gravitational pull upon the rotor from right to left dependent uponthe inclination of the module, and

a force from left to right produced by the pressure difference acrossthe balance piston 317.

The diameter D may be chosen in design so that the axial force producedat the balance piston 317 offsets as great a part as is practicable ofthe resultant of the other axial forces.

Another embodiment is illustrated in FIG. 4 that is a modified versionof the embodiment of FIG. 3. To make the distinction between moving andstationary parts evident, the stationary parts are hatched in the upperpart of the figure.

FIGS. 3 and 4 may be related one to the other by the element 410 thatcorresponds to the right hand journal bearing 310 of the compressorshown in FIG. 3. In the embodiment of FIG. 4, the auxiliary compressorto the right of the bearing is a two stage centrifugal compressor asopposed to the two stage axial compressor of the embodiment of FIG. 3.

With other things equal the pressure rise across a centrifugal and anaxial flow compressor stage is set by the peripheral speed of thecompressor disk, and by the mean peripheral speed of the runner bladesof the axial flow stage.

When confined within the same diameter casing, an axial flow stage mayhave a greater mean diameter of its runner blades than the outerdiameter of the centrifugal compressor disk because the centrifugalcompressor requires a diffuser outboard of its disk, and the axial flowcompressor does not. This consideration with relation to FIGS. 3 and 4may lead to a single stage axial auxiliary compressor in the embodimentof FIG. 3 performing the same duty as the two stage centrifugalcompressor of FIG. 4.

FIGS. 5 a and 5 b are idealized enthalpy-entropy diagrams for the gasflows compressed by the auxiliary compressors of the embodiments ofFIGS. 3 and 4, and then throttled to their initial pressures in thebearings.

With reference to FIGS. 3 and 5 a, the gas flows in to the module atpressure P1. Downstream of the running blades of the main compressor, inthe channel Y, the gas is at pressure P3, and after passage through theauxiliary compressor it enters the bearings at pressure P4. The gas isthen throttled down to the pressure P1 at its exhaust from the bearings.

Constant pressure lines for P1 and P4 are drawn in FIG. 5 a. The inflowof gas occurs at ‘a’, the gas is compressed to ‘b’ and then throttled toits outflow at ‘c’. The inflow is of relatively cool gas, and theoutflow is gas heated by the energy input of compression over ‘a’ to‘b’.

If provision is made by means of a heat exchanger to cool the same gasflow from ‘c’ to ‘a’ then the gas for the bearings would be a closedcircuit. Once purified the same gas would be in continuous use. FIGS. 6a and 6 b illustrate diagrammatically an embodiment in which such aclosed circuit is provided for the high-pressure gas.

In the embodiment of FIG. 6 a the main compressor is a two-stage axialflow compressor shown at 614, 613, 612 and 604. A cylindrical baffle 603b with the casing of the motor 603 a form a channel Y in which gas flowsover the cooling fins Y1 of the stator of the motor. Channel Y, andchannel X become a single channel downstream of the baffle.

The closed circuit will be now described, taking the volume T as itsstarting point. Gas from T flows through the filter 620 in to the intakeof the axial flow compressor 616. The compressor delivers high pressuregas in to the volume U and from there it passes via ports 619 to thejournal bearing 610, and to the journal and thrust bearing at 609 viapipes of which one is at L1. The gas is throttled on passing through thebearings and exhausts in one instance first to the volume V, and thenvia the air gap of the motor to volume W where it joins the exhaust fromthe other bearing. The gas is returned to the volume T via pipes ofwhich one is indicated at L3. Pipes L3 are laid in the channel X wherethe passing of the main flow of gas past them will cool the pipes andthe circulating gas within them.

There is also a leakage flow of high-pressure gas from the volume U tothe volume V via the labyrinth gland 618. This leakage through thelabyrinth is a parallel path in which the gas is throttled down to thesame lower pressure as the high pressure gas that is passed through abearing.

The only connection of the closed circuit to the main gas flow is byleakage through the labyrinth gland 612 a. This leakage will equalizethe pressures either side of the labyrinth, and consequently the lowpressure of the closed circuit will be the pressure P3 downstream of thesecond stage runner blades of the main compressor. FIG. 5 b is theenthalpy-entropy diagram of the closed circuit.

With reference to FIG. 5 b, the cooling of the gas from ‘b’ to ‘c’depends upon the effectiveness of heat transfer across the tube L3. Abalance has to be made between the energy input into the circulating gasby the auxiliary compressor, and the heat lost from the circulationthrough the walls of pipes L3 to the main gas stream. The balance iscreated through the temperature of the circulating gas. The gas losesmore heat across the walls of the pipes L3 as the gas temperature rises,and at the same time the energy input in to the gas by the compressorfalls. The gas of the closed circuit will be at the temperature at whichheat loss and energy input are in balance. It is desirable that thetemperature of the gas at the inlet of the auxiliary compressor shouldbe brought as close as is practicable to the temperature of the flow inthe channel X by optimizing the gas to gas heat transfer coefficient ofthe wall of pipes L3.

The flow of gas into or out of the closed circuit through the labyrinth612 a is so minimal that the danger recedes of the bearings becomingdamaged by particulate matter. It is likely that any particulate matteroriginally borne by gas entering the closed circuit via the labyrinth612 a will have previously been centrifuged by virtue of the whirlcomponent imparted to the gas by the bladed impeller wheel.

The flow resistance in the combined channels X and Y is increased by theintrusion of pipes and fins in to the flow area. For that reason, themain compressor 604, 612, 613, 614 has been changed for illustrativepurposes from the compressor of FIG. 3 to a two-stage compressor.Whether such a change is needed can only be determined in eachparticular instance from a design study.

The auxiliary compressor 616 of FIG. 6 a is a single stage compressor incomparison with the two stage auxiliary compressor of FIG. 3.

The section A-A of FIG. 6 a outboard of the motor casing is illustratedin FIG. 6 b. The cooling fins of the stator are at Y1 between the casingof the motor 603 a and the baffle 603 b. The four pipes L1 run betweenadjacent fins. Eight pipes L3 are illustrated equally spaced around thecircle in the channel X. The pipes L3 may conveniently be formed as anextrusion with both internal and external fins to enhance the gas to gasheat transfer.

Although a specific embodiment of the invention has been disclosed, itwill be understood by those having skill in the art that changes can bemade to this specific embodiment without departing from the spirit andscope of the invention. The scope of the invention is not to berestricted, therefore, to the specific embodiment, and it is intendedthat the appended claims cover any and all such applications,modifications, and embodiments within the scope of the presentinvention.

1. A compressor designed to be lowered into a well of a natural gasreservoir to assist in extracting gas from the reservoir, the compressorcomprising: at least one casing; at least one rotor mounted within thecasing; at least one electric motor for driving the rotor; one or moregas bearings supporting the rotor for rotation relative to the casing,the gas bearings being arranged at an upstream end and a downstream endthereby arranged at opposite ends of the motor; at least one bladedimpeller wheel for compressing a production of gas from a reservoirwhich is mounted on an overhanging end of the rotor that projects beyondeach of the gas bearings at the upstream end of the motor; at least oneauxiliary compressor mounted on the downstream end of the rotor so thatthe auxiliary compressor draws from down stream the bladed impellerwheel and pressurizes the gas before supplying the gas to the bearingsof the rotor; wherein all the gas bearings of the auxiliary compressorand of the electric motor are arranged on a same side of the bladedimpeller wheel; and during operation, the gas flows over to cool theelectric motor.
 2. The compressor of claim 1, wherein the rotor isformed hollow to assist in cooling of the motor.
 3. The compressorsystem of claim 1, further comprising at least one additional auxiliarycompressor arranged in tandem with the auxiliary compressor.
 4. Thecompressor of claim 1, wherein the the casing includes a channel formedtherein which runs parallel to the rotor between an output of theauxiliary compressor mounted at the downstream end of the rotor up andthe upstream end of the rotor for supplying the gas to the bearings ofthe rotor.
 5. The compressor of claim 4, wherein the auxiliarycompressor is also an axial compressor.
 6. The compressor of claim 4,wherein the auxiliary compressor is a centrifugal compressor.
 7. Thecompressor of claim 4, further comprising: a purifier is provided in anintake of the auxiliary compressor.
 8. The compressor system of claim 4,further comprising at least one additional auxiliary compressor arrangedin tandem with the auxiliary compressor.
 9. The compressor of claim 4,wherein the gas pressurized by the auxiliary compressor is dischargedinto an axial flow of produced gas after passing through the bearings.10. The compressor of claim 9, further comprising: means fortransferring heat from the gas discharged from the bearings to the axialflow of the gas and for recycling a cooled gas to an intake of theauxiliary compressor, whereby the gas supply to the bearings flowsthrough a substantially closed circuit.
 11. The compressor of claim 4,wherein both the main compressor and the auxiliary compressor areoverhung with all the bearings being situated axially between the maincompressor and the auxiliary compressor.
 12. The compressor of claim 11,wherein the auxiliary compressor is also an axial compressor.
 13. Thecompressor system of claim 12, further comprising at least oneadditional auxiliary compressor arranged in tandem with the auxiliarycompressor.
 14. The compressor of claim 11, wherein the auxiliarycompressor is a centrifugal compressor.
 15. The compressor of claim 14,further comprising: a purifier is provided in an intake of the auxiliarycompressor.
 16. The compressor of claim 15, wherein the gas pressurizedby the auxiliary compressor is discharged into an axial flow of producedgas after passing through the bearings.
 17. The compressor of claim 16,further comprising: means for transferring heat from the gas dischargedfrom the bearings to the axial flow of the gas and for recycling acooled gas to an intake of the auxiliary compressor, whereby the gassupply to the bearings flows through an essentially substantially closedcircuit.
 18. The compressor system of claim 17, further comprising atleast one additional auxiliary compressor arranged in tandem with theauxiliary compressor.
 19. The compressor system of claim 17, furthercomprising: a plurality of auxiliary compressors arranged in tandemposition at different heights along a bore hole of a well.
 20. Adownhole compressor comprising: at least one rotor with at least adownstream gas bearing and an upstream gas bearing mounted thereon; atleast one casing for supporting the downstream gas bearing and anupstream gas bearing of the rotor so as to permit rotation of the rotorrelative to the casing, wherein the casing includes a channel formedtherein which runs parallel to the rotor between and the downstream gasbearing and the upstream gas bearing; at least one overhanging bladedimpeller wheel for compressing a gas, the overhanging bladed impellerwheel mounted on an upstream end of the rotor that projects beyond allthe upstream gas bearing; at least one auxiliary compressor mounted forfurther compressing the gas, the auxiliary compressor mounted on adownstream end of the rotor that projects beyond all the downstream gasbearing; at least one electric motor disposed on the rotor between theupstream gas bearing and the gas downstream, whereby the motor and thedownstream gas bearing and the upstream gas bearing are all situated inbetween the overhanging bladed impeller wheel and the auxiliarycompressor so that the gas flows from the overhanging bladed impellerwheel to cool the electric motor and afterwards at least a portion ofthe gas flows through the auxiliary compressor into the channel in thecasing in a direction towards the upstream end of the rotor to supplythe gas to the upstream gas bearing and the downstream gas bearing.